1. Field of the invention
The present invention relates to an ink recording apparatus for use in printers or the like. It is to be noted that the word `recording` herein used refers to the fact that any desired patterns of characters, symbols, or the like are written down onto a printed material such as paper with ink jetted out by an apparatus of the present invention.
2. Description of the related art
A conventional ink recording apparatus is shown in the Japanese magazine "Nikkei Mechanical", issued on May 29, 1989, pp. 90 to 91, the apparatus exemplifying such ink recording apparatus that are currently used in printers featuring compactness suitable for office or personal use thereof.
FIG. 11 shows a construction of such a conventional ink recording apparatus. In the figure, a slit plate 1 is provided with a plurality of slits 2 having a width of 50 .mu.m and a length of 8 mm in place of nozzles. The slit plate 1 has also a plurality of auxiliary holes 3 equal in number to a plurality of heating elements 5 formed on a base plate 4, with an ink reservoir 6 as well provided at the slit plate. On the base plate 4 there are formed a plurality of electrodes 7 in correspondence to the heating elements 5 and moreover a plurality of fluid resistance elements 8 shaped into a long, narrow protrusion. Besides, between the slit plate 1 and the base plate 4 there is disposed a spacer 9, which in conjunction with the slit plate 1 and base plate 4 defines a portion serving as an ink chamber 11 illustrated in FIGS. 12a to 12d. Under the base plate 4 there is provided an ink tank 10, whereon all the units are piled up to make up a head. The heating elements 5 and formed by piling up a glass layer, resistors, electrodes, and a protective coat on the base plate 4, as in a common thermal head.
A conventional ink recording apparatus having a construction as described above will jet ink droplets while overcomes steps as shown in FIGS. 12a to 12d. Each step is detailed below:
(a) First, when pulse voltage is applied to the heating elements 5 on the base plate 4 to heat the ink contained in the ink chamber 11, the ink in the vicinity of the heating elements 5 vaporizes to make a large number of small bubbles 12;
(b) Second, the small bubbles 12 merge together and grow into a larger bubble 13 that overcome the surface tension, causing ink swells to be produced at the slits 2;
(c) Third, when the heating elements 5, on completion of heating, are cooled down to stop the bubble 13 from being produced, the swelling of ink is intercepted to produce ink droplets 14; and
(d) Finally, the ink droplets 14 are jetted out through the slits 2 by the power of growing bubble 13.
If a number of heating elements 5 share the slits 2 and the ink chamber 11 with one another as in the above conventional apparatus, there arises a problem that the ink droplets 14 derived from adjoining heating elements 5 may interfere with each other. In the conventional apparatus, however, the fluid resistance elements 8 provided between adjoining heating elements 5, 5, as shown in FIG. 11, will serve to prevent pressure waves from being horizontally propagated while the bubbles are being produced, thereby allowing the ink droplets 14 to be formed and jetted out without being adversely affected by such pressure waves. Furthermore, the auxiliary holes 3 provided to the slit plate 1 will absorb the pressure waves, so that pressure waves may be prevented also from being reflected.
In the conventional apparatus arranged as described above, since the apparatus can not vary the size of ink droplets, it is forced to perform pseudo area gradation recording by signal a process instead of area gradation recording. The result is that the roughness in the picture quality is noticeable due to dither and the like. Accordingly, the conventional apparatus produces inferior picture quality compared to an apparatus employing another recording system, e.g. a heat sublimation system type printer which uses a sublimation type dye. Although, it is possible to inject ink droplets at a point, one over the other, it takes a long time to record one dot when a dynamic range is set to be wide. As a result, the high speed recording performance of the ink jet system is poor.